Making it easier for repository rules to test for presence of rules_license

[Tony Aiuto]

I started on the rules_jvm_external update, and soon hit a painful dead end at jvm.bzl in @bazel_tools.

That prompted me to start a discussion about a more general feature to query on having a repository available, so downstream (in the WORKSPACE) rules and optionally make use of those features.

https://github.com/bazelbuild/bazel/discussions/18473

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